Wearing tight underwear can lead to the decrease in size of your penis. In 1745 a custom was established where men would wear tight underwear so that their penises would not bulge, since back then It was rude and disrespectful to have a long penis.

No tight underwear does not cause any problems with your penis, testicles or your sperm. Even if your testicles get too warm to make sperm apparently there is no long term damage done as as soon as the temperature drops to the correct level they work perfectly again. Back in the 1960s to 1990s most males wore firm fitting underwear and very few had any problem with overheating of the testicles cutting back their sperm production.
It is quite safe to wear tight underwear and it will cause no penis shrinking or turn you sterile as some people stupidly claim. If someone has told you that it can cause problems likely they are either terminally stupid or trying to fool you.